They're (mostly) the same channels shared by all of them. It's not like an ethernet address (where everyone on the network speaks the same protocol and just ignores packets not for it), it's a radio frequency timeslice that's allocated to you, and you and your nearby tower can speak any radio modulation they want in that timeslice, even ones that other phones or other towers don't support.

However new technologies sometimes also bring new spectrum blocks like mmwave 5G.

> Right, so if I'm still rocking 4G only while 'everyone' else in the area is using 6G with some other stragglers on 5G, ...

You will keep receiving service until it becomes unprofitable to do so, same as always, but that time will be a lot further down the road than it used to be because there's basically no reason to care outside of congested cells where making the most of every bit of airtime matters.

If you're in a congested area, prepare for newer tech to be prioritized, often legitimately because the newer modes tend to be better at sharing the air and thus more users can be better served by prioritizing them on limited spectrum.